Worldwide data suggest that migrants are at increased risk of HIV infection. The high rates of HIV infection and the mobility of migrant populations make migrants a living bridge to spread the AIDS virus between the countries/places of origin and the countries/places of destination. Research findings indicate that the increased HIV infection among migrants is associated their high prevalence of HIV risk behaviors (substance use and risky sex), which in turn, are determined by socioenvironmental and situational factors. However, little is known about the mechanisms by which environmental factors lead to HIV risk behavior. Guided by the Social Capital Theory, in this project, we propose to test two hypothesized mechanisms that may link social influences to HIV risk behavior among migrants - the lack of social support (including informational, instrumental and emotional) and the weakening of informal social control (including collective efficacy and normative beliefs). Rural migrants lost their social capital (defined as durable, resource-rich, trustworthy, and reciprocal network connections a person accumulated) deeply rooted in the place of rural origin and they may experience difficulties rebuilding social capital in the urban destination due to their low socioeconomic status. Migrants with limited social capital may then experience difficulties obtaining informational, instrumental, and emotional support to establish their career and life in the stressful urban settings; consequently use of substance becomes an attractive and convenient option. The lack of social capital may also make migrants less responsive to the social capital-based collective efficacy (volunteer actions by community members against deviate behaviors that may devalue the community), increasing the likelihood to engage in sexual risk behavior. When rebuilding social capital, migrants may adopt more liberal attitudes and more open values about sex, leading to sexual risk behavior. Exposure to more opportunities in urban settings will further exacerbate the likelihood. The low SES of female migrants may explain the higher HIV risk among them than among their male counterparts. The unprecedented rural-to-urban migration in China with the growing HIV/AIDS epidemic and the high prevalence of HIV risk behavior among rural migrants provide a window opportunity to investigate these issues. We will conduct the proposed research in Wuhan, China, a metropolitan city and provincial capital with a population of 8.1 million and one million rural migrants. The proposed research is highly significant and very innovative. Data for the proposed research will be collected using probability samples. The study samples will be selected using cost-effective methods supported by the GIS/GPS technology and a sample of 1200 rural migrants will be contrasted with the same number of non- migrant residents in the rural areas from where rural migrants originate and non-migrant residents in the urban areas where the rural migrants are currently living and working. In addition to survey data, sexually transmitted infections will also be assessed. We expected that findings from this research will advance our understanding of the migration-related HIV/AIDS transmission and provide needed data supporting our next step of research - devising and implementing innovative programs for HIV prevention.